Three-dimensional architecture for solid state radiation detectors

ABSTRACT

A radiation-damage resistant radiation detector is formed on a substrate formed of a material doped with a first conductivity type dopant. The detector includes at least one first electrode formed of first conductivity type dopant, and at least one second electrode that is spaced-apart from the first electrode and formed of a second conductivity type dopant. Each first and second electrode penetrates into the substrate from a substrate surface, and one or more electrodes may penetrate entirely through the substrate, that is traversing from one surface to the other surface. Particulate and/or electromagnetic radiation penetrating at least a surface of the substrate releases electrons and holes in substrate regions. Because the electrodes may be formed entirely through the substrate thickness, the released charges will be a relatively small distance from at least a portion of such an electrode, e.g., a distance less than the substrate thickness. The electrons and/or holes traverse the small distance and are collected by said electrodes, thus promoting rapid detection of the radiation. By providing one or more electrodes with a dopant profile radially graded in a direction parallel to a substrate surface, an electric field results that promotes rapid collection of released electrons and said holes. Monolithic combinations of such detectors may be fabricated including CMOS electronics to process radiation signals.

Said provisional application was developed with support from the U.S.Department of Energy under grant DE-FG0394ER40833, and the U.S.government may have rights to the within application.

RELATION TO PREVIOUSLY FILED APPLICATION

This is a continuation application from Provisional Patent applicationSer. No. 60/011,388 filed Feb. 8, 1996, entitled "3D-A NEW ARCHITECTUREFOR SOLID STATE RADIATION DETECTORS", from which priority is claimed.

FIELD OF THE INVENTION

The present invention relates to solid state radiation detectors, andmore specifically to detector architecture that promotes fasterdetection response.

BACKGROUND OF THE INVENTION

Solid state radiation detectors are known in the art, and provide auseful mechanism for detecting radiation. Such detectors have evolvedfrom detectors using surface barrier electrodes, to ion-implantedelectrodes.

Descriptions of such detectors and their uses may be found inapplicant's article "A Proposed VLSI Pixel Device for ParticleDetection", Nucl. Instr. and Meth. A275, 494 (1989), A342 59-77 (1994),and in U.S. Pat. Nos. 4,593,381, 5,237,197, 5,355,013, 5,461,653 and5,465,002, among other references.

FIG. 1 depicts such a prior art radiation detector 10, which is morefully disclosed in U.S. Pat. No. 5,237,197 (in which applicant herein isco-inventor). In FIG. 1, a detector array 10 includes a preferablylightly doped P-type charge depletable substrate 20, having first andsecond surfaces 30 and 40 spaced-apart by a substrate thickness L ofperhaps a few hundred microns. Substrate thicknesses in this rangeprovide good sensitivity for collecting radiation-generated charge fromwithin the substrate, as well as providing acceptable voltage breakdownlevels, and protection from radiation damage.

Adjacent the first substrate surface 30 voltage-biasable doped wellregions 50 of preferably N-type material are formed. Buffer well region55 is formed of N-type or P-type material, depending upon the nature ofthe circuitry 60 in this well region. Well regions 50, 55 preferably aresufficiently highly doped to act as an electrostatic shield forunderlying regions of the detection device. Electronics 60 may befabricated within buffer well region 55.

Also adjacent first substrate surface 30 and separated from each otherby the N-type well regions 50 are formed spaced-apart collectionelectrodes 70, preferably made from highly doped P-type material.Preferably the gate lead of one (or more than one)metal-oxide-semiconductor ("MOS") transistor 80 is coupled to eachcollection electrode 70. The lower surface of the substrate includes apreferably heavily doped N-diffusion region 90, beneath which is anelectrode (not shown), and isolation regions 100. Of course, theconductivity types of the materials used to form detector 10 could bereversed, e.g., substituting P-type for N-type and vice versa.

One collection electrode 70, its associated MOS device 80, and indeedthe associated underlying semiconductor structure may collectively betermed a "pixel", and the terms pixel and detector may be usedinterchangeably. It is seen from FIG. 1 that P-type collectionelectrodes 70 and P-type substrate 20 form a plurality of PN diodejunctions with the N-type well regions 50 adjacent the first surface.

In practice, a well bias voltage of many volts is coupled between thecollection electrode regions and bottom regions and N-doped wellregions. The resultant electric fields extend from the second surface 40toward and to the first surface 30. The resultant depletion regionextends through the perhaps 300 μm thickness of the substrate, whereupona plurality of P-I-N diodes are formed by P-type collection regions 60,intrinsic substrate region 20, and N-type region 90.

The biasing causes force lines to emanate from the N-diffusion region 90through the substrate thickness and focus upon the P-type collectionelectrodes 70. Incoming radiation (not shown) releases charge within thesubstrate, which charge is focused by the force lines and caused to becollected by the electrodes 70. As noted, N-wells 50 further serve as aFaraday shield for the array of pixels in structure 10. As noted, wellregions 50, 55 can also serve as areas in which electronics arefabricated. Unfortunately, CMOS electronics that require wells of bothdopant conductivity types can present a problem. Such CMOS electronicscan be accommodated in the area over the active detection region,providing wells of like-conductivity type as the collection electrodesare implanted completely within wells of the opposite conductivity type.Understandably, if same-type wells were to be formed directly on thedepleted silicon substrate, the wells would collect ionization charge onthe substrate, which charges would not be collected and detected by thecollection electrodes and assorted circuitry.

But as noted in the U.S. Pat. No. 5,237,197 patent, detector 10 cannonetheless function reasonably well because the wells surrounding thecollection electrodes were doped with opposite type dopant and wereback-biased relative to the collection electrodes. This configurationcaused electric field lines to be directed to carry one sign ofionization charge from the substrate and the well to the collectionelectrodes. Like-signal wells, needed for CMOS electronics, were placedalong the structure edges, beyond the sensitive detection area. Eventhough only perhaps 10% of upper surface 30 may be covered by collectionelectrodes, efficiency in the sensitive regions is extremely high withmore than 99.99% of the radiation-induced charges being collected byelectrodes 70. The collection electrodes preferably are uniformlydistributed in a two-dimensional array on the surface, to provideresultant uniform array sensitivity and spatial resolution.

Once the radiation-induced charge has been collected by the collectionelectrodes 70, the transfer to an associated MOS device(s) can be rapidas the distance is now but a few microns. Further, because there issmall capacitance (C) at the MOS gate, the charge (q) developed by theincoming X-ray radiation can produce a substantial voltage signal (v),since v≈q/C. Electronics 60 may be used to signal process the chargeassociated with the MOS devices. For example the collected charge at aMOS gate may be used to modulate readout current caused to flow throughthe MOS device. Such readout may be made on an addressable row-columnbasis.

As noted, radiation detection sensitivity for prior art detector 10 canbe very high. But radiation-induced charge cannot be detected until ithas been collected by the surface-located collection electrodes 70.Unfortunately the collection or drift path that released charge musttraverse before being collected can be very long, e.g., comparable tothe few hundred micron full substrate thickness. Of course should someradiation-generated charge happen to be released closer to thecollection electrode surface, collection of the charge can occur in ashorter time. In practice, prior art detectors using two-dimensionalelectrodes such as shown in FIG. 1, or the common silicon striptechnology that preceded what is shown in FIG. 1, may take upwards of 25ns for charge-produced signals to return to a baseline level from apeak. Of course, amplifier delays may extend this time even further.

Attempting to reduce radiation detection time by using a thinnersubstrate is counter productive because thinner substrates have shortertracks, and therefor less signal charge. Also, thinner materials canbreak more readily during fabrication. It would also be desirable toprovide a detector structure that is kept small in size in the presenceof radiation damage, requiring a smaller voltage magnitude to achievedepletion, while still preventing so-called bulk type-reversals.Finally, in many detection environments it is necessary to continuouslyrefrigerate the detector, even for maintenance.

Thus, there is a need for a sensitive solid state radiation detectorhaving improved detection response times. Such detector should alsoprovide good voltage breakdown, not require excessively high depletionvoltages, and exhibit good radiation damage resistance characteristics.Further, such detector should permit implementing a monolithiccombination of collection electrodes and CMOS electronics, withoutthereby hindering collection of released charge. Finally, such adetector should function without the need for operation at lowtemperatures.

The present invention provides such a detector, and a method for itsfabrication.

SUMMARY OF THE INVENTION

A three-dimensional solid-state radiation detector is fabricated on asubstrate formed of a material doped with a first conductivity typedopant, whose first and second surfaces are separated by the bulkthickness, which may be several hundred microns. The detector includesat least one first electrode formed of the same first conductivity typedopant and penetrating from one of the surfaces at least partially intoif not completely through the substrate bulk. The detector also includesat least a one second electrode that is spaced-apart from the firstelectrode. The second electrode is formed of an opposite conductivitytype dopant and penetrates from either of the surfaces at leastpartially into if not completely through the substrate bulk.

The three-dimensional electrodes preferably penetrate entirely throughthe substrate bulk. As a result, detectable particulate and/orelectromagnetic radiation that penetrates at least one substrate surfaceand releases electrons and holes in substrate regions, releases them ata relatively small distance from at least a portion of one of theelectrodes. Accordingly, radiation-induced electrons and/or holes onlyhave to traverse a small distance before being collected by anelectrode, which enables rapid detection of the incoming radiation.Electrons and/or holes traverse the small distance and may be collectedby the electrodes an order of magnitude faster than if a prior arttwo-dimension electrode detector were used.

The electrodes may be formed by first defining electrode column-likeholes in the substrate, and then filling the holes and making themconductive, for silicon-based detectors, preferably with silane-producedpolysilicon that is doped. Electrodes with a dopant profile radiallygraded in a direction parallel to a substrate surface will have anelectric field that promotes self-depletion and rapid collection ofreleased electrons and holes. Alternatively, first electrodes might beformed using a suitably high energy ion implanter to implant firstconductivity type dopant through openings in a first mask. Next,opposite conductivity type dopant is implanted through openings in asecond mask to form the second electrodes.

An externally applied potential normally will be coupled between thefirst and second electrodes to further promote substrate depletion andto increase the inter-electrode electric field magnitude present to helpguide release charge to adjacent electrodes for collection.

The disclosed three-dimensional architecture advantageously permits amonolithic combination of collection electrodes and CMOS electronics.The well structure for the CMOS electronics is such that substantiallyall charge is collected by the collection electrodes. As a result, onlya small fraction of charges remain to be collected by the wellstructure, and thus lost for measurement purposes.

Other features and advantages of the invention will appear from thefollowing description in which the preferred embodiments have been setforth in detail, in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a radiation detector, according to the prior art;

FIG. 2A depicts a monolithic structure with radiation detectors withthree-dimensional collection electrodes, according to the presentinvention;

FIG. 2B depicts a single cell portion of a basic radiation detector,according to the present invention;

FIGS. 3A-3D depict equipotentials for one-quarter of the unit cell ofFIG. 2B for different dopant concentrations and applied potential,according to the present invention;

FIG.4 depicts drift lines for a quarter-cell configuration for 5 Vpotential and 10¹² dopant atoms/cc, according to the present invention;

FIGS. 5A-5C depict electric field magnitudes along different lines for aquarter-cell configuration, according to the present invention;

FIG. 5D is a comparison of electric field magnitudes for a quarter-cellconfiguration for different substrate dopant concentrations, accordingto the present invention;

FIG. 6 depicts lines of equal drift time for potential distributions fora quarter-cell, according to the present invention;

FIGS. 7A-7L depict charge density contours for electron-hole pairs in aquarter-cell having 10 V potential, 10 ¹² /cc dopant concentration,according to the present invention;

FIGS. 8A and 8B depict current pulses on collection electrodes fromtracks parallel to the electrodes, according to the present invention;

FIGS. 9A-9F depict oxide interface charge effects under varyingconditions for a two-dimensional model;

FIGS. 10A, 10B, 10C depict top views of various detector structures,according to the present invention;

FIGS. 10D, 10E, 10F, 10G, and 10H are side views of various detectorstructures, according to the present invention;

FIG. 11 depicts equipotentials for the cell shown in FIG. 10A, accordingto the present invention;

FIG. 12A depicts electric field magnitudes for the cell of FIG. 10Aalong a line connecting adjacent P+ and N+ electrodes, according to thepresent invention;

FIG. 12B depicts electric field magnitudes for the cell of FIG. 10Aalong a horizontal line through the middle of the equipotential graph ofFIG. 11, according to the present invention;

FIG. 13 depicts equipotentials for the cell of FIG. 10C, according tothe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides architecture for solid-state radiationdetectors in which a three-dimensional array of electrodes penetratesinto and possibly entirely through the substrate bulk. The resultantstructure reduces charge collection distances and calculated collectiontimes by about one order of magnitude relative to what is realized usingprior art planar technology strip and pixel detectors with detectorsurface electrodes. In addition, the three-dimensional architecturepermits use of depletion voltages that are about one-to-two orders ofmagnitude lower that what is required for prior art detectors. Further,the disclosed architecture enables maximum substrate thickness, often animportant consideration for x-ray and gamma-ray detection, to beconstrained by the electrode length, rather than by material purity ordepletion-depth limitations due to voltage breakdown.

A three-dimensional solid-state detector according to the presentinvention has increased resistance to radiation damage. As such, thedetector is especially useful in environments such as high intensitycolliders, wherein prior art detectors would experience severe bulkradiation damage rendering them inoperable.

FIG. 2A depicts a monolithic structure embodiment of a radiation-damageresistant radiation detector 170 according to the present invention.However, it is to be understood that non-monolithic detector systems mayalso be realized with the present invention. Detector 170 is formed on apreferably lightly doped P-type charge depletable substrate 180 havingspaced-apart first and second surfaces 190 and 200 and a substratethickness L that is perhaps a few hundred microns. In contrast to thetwo-dimensional electrode array of prior art FIG. 1, note in FIG. 2Athat P-type electrodes 210 descend from P-implants (or wells) that serveas collection electrodes 75 formed from the upper substrate surface 190substantially into if not completely through the substrate thickness tothe lower surface 200. Note too that N-type electrodes 222 descend fromN-wells 50 formed at the upper substrate surface and descendsubstantially into if not completely through the substrate thickness.CMOS type circuitry may advantageously be disposed within well 50 and/orwells 55 (it being understood that wells 55 may include P-type, and/orN-type structures, depending upon the nature of the electronics to beincluded), to measure charge from collection electrodes, e.g., 210, aswell as to signal process resultant charge signals.

FIG. 2B depicts a quarter-cell portion of a unit basic radiationdetector such as detector 170. For clarity in FIG. 2B, detector 170 isformed on a substrate bulk 180 having spaced-apart first and secondsurfaces 190 and 200. Detector 170 is shown as though cut in halflengthwise, with the two halves slightly pulled apart to provide abetter understanding of the structure. Substrate thickness L is perhapsa few hundred microns, and the X and Z dimensions of the unit detectorcell shown in FIG. 2B may be in the range of perhaps 20-100 μm. Ofcourse different X, Y, and/or L dimensions may be used.

Extending into if not through the detector bulk are one or moreelectrodes, including a preferably P+doped electrode 210, andN+electrodes shown as 220, 222, 224, 226, 228, 230, 232, and 234. Theelectrode spaced-apart distance or pitch typically may range from about10 μm at present (and likely 5 μm or less in a few years as betterfabrication equipment is available) up to a distance comparable tosubstrate thickness L. The electrode depth may be a small fraction ofthe total substrate thickness L, perhaps 5%, but preferably theelectrode depth (or length) is equal to the entire substrate thickness,e.g., 100%. Through-substrate electrode lengths are preferred as theresulting short charge collection distances provide fast chargecollection while permitting use of low depletion voltages.

The quarter-cell configuration of FIG. 2A was examined as to voltagedistributions, using MEDICI modeling software available from TechnologyModeling Associates Inc., of Palo Alto, Calif. Calculations were alsomade using a sequential-over-relaxation method described by K. Binns andP. Lawrenson, "Analysis and computation of electric and magnetic fieldproblems, (Pergamon, 1973) p. 241. (For the latter analysis, symmetricboundary conditions (Vi+1, j,k)=v(i-1,j,k) were used for the (cubic)cells, on either side of a boundary at a plane i=constant, where Vrepresents voltages at cell centers. At silicon-insulator boundaries, anext voltage for any cube is found from the average of the fouradjoining voltages on the boundary and the adjoining voltage furtherinto the silicon (plus the usual term from fixed charges). This reflectsthat in equilibrium, there is no net charge transport into the cube.Thus, the sum of current across the five cell faces, as well as the fivevoltage differences, are zero. In this approximation, surface currentswere neglected, and effects of induced charges were calculated usingRamo's theorem as set forth in S. Ramo, "Currents induced by electronmotion", Proc. of the I.R.E., 27 (1939) 584. Coulomb forces wereapproximated by subdividing 24,000 electron-hole pairs from a typicalminimum-ionization track into packets of 40 charges each, which diffusedand drifted as a group. Results did not change significantly when thepacket size was changed.

Of the two computation methods, presently only MEDICI can calculatefields and current flow in the presence of surfaces and undepletedsilicon. When both methods could be used to analyze the same problem,results generally agreed.

Depletion voltages for the sample diode shown in FIG. 2B were 1.6V,1.8V, 3.8V, and 8.8V for dopant concentrations of 10¹², 3×10¹², 10¹³,and 3×10¹³ dopant atoms/cc., including a contribution from the built-involtage at the electrodes ranging from about 0.7V to 0.8V. The valuesare not proportional to the dopant concentrations because in the courseof fully depleting the lightly doped silicon substrate, part of theheavily doped region around the electrodes is also depleted. Applicant'sprototype pixel detectors had a doping concentration of 1.2×10¹² dopantatoms/cc. It is anticipated that a 10 year exposure for such pixeldetectors in a potentially bulk-damaging environment would not increasedoping much beyond about 10¹³ dopant atoms/cc.

As noted, because the path from a radiation-induced charge to a nearestcollection electrode can be substantially shorter than the distance L,collection times for detector 170 are rather fast. Rapid collection timeis especially useful in a quantum mammography system proposed byapplicant, in which individual x-ray radiation hit locations arerecorded. Further, the dual benefits of fast collection and lowdepletion voltage will be particularly useful in high luminositycollider environments, in which prior art detectors face severe problemsboth from high event rates and from increased depletion voltages due tobulk radiation damage. Use of detection devices according to the presentinvention will advantageously eliminate the need for high depletionvoltages, and the need to refrigerate the detector continuously, evenduring maintenance.

Fabrication of a detector according to the present invention can becarried out as follows. Initially column-like holes are formed insubstrate 180. These holes penetrate from the substrate surface 190 atleast partially into the substrate bulk, and preferably entirely throughthe substrate bulk, e.g., through lower bulk surface 200. Preferablythese holes define cross-sectional dimensions having a transversedimension ranging from about 0.5 μm to about 25 μm. As noted, thecolumnar length of these hole openings may vary from about 5% topreferably 100% of the substrate thickness.

Aside from fabrication equipment capabilities, other factors should alsobe considered in deciding diameter (or transverse dimension) of theholes to be formed. On one hand, the electrode will exhibit an electrodecapacity (C) that advantageously is reduced with smaller diameters. Notethat smaller diameter electrodes will decrease any inefficiency incollection of ionization charge. However, on the other hand, electroderesistance and maximum electric fields disadvantageously increase withelectrodes formed in smaller diameter holes.

For a 300 μm long electrode, the capacity C is about 0.1 pF. Theelectrode (resistance)×(capacitance) or RC product is related to theleast time required for pulses to leave the electrodes. RC is about 90ps for N+ electrodes, and is about 225 ps for P+ electrodes. For some(but not all) detector structures, the magnitude of R may also play arole in the noise performance, depending upon sensitivity of theelectronics to which the electrode is coupled. If especially lowresistance electrodes were required, one could selectively deposittungsten in the central core of the holes. However, such measures arenot deemed necessary for pixel detectors using three-dimensionalelectrodes, as disclosed herein.

To a first approximation, the initial signal developed on the electrodesdue to released charge (q), q/^(C) electrode' is independent of waferthickness (L) for penetrating ionizing particles, as electrodecapacitance C_(electrode) is approximately proportional to thethickness. In later stages of some electronic readout systems thinnerwafers may produce smaller signals. However, any degradation ofsignal-to-noise ratio will be less rapid using detectors withthree-dimensional electrodes, than would be the case for prior artplanar two-dimensional electrode structures, as shorter electrodes havelower capacitance. If thinner substrate detectors are deemed desirable,perhaps due to multiple Coulomb scattering considerations, it is likethat fabrication equipment and wafer handling considerations will setthe lower limit on substrate thickness. However, thinner wafers shouldactually permit smaller hole diameters to be fabricated, resulting in adecrease in C_(electrode).

Having decided upon suitable dimensions, the holes for the electrodesare formed preferably using deep reactive-ion etching ("RIE"), a processthat can provide holes having depth-to-width ratios of 15:1 or greater.

In practice, etch-formed holes having a diameter of 15 μm and a lengthof 200 μm, with a top-to-bottom taper in the resultant hole-column ofless than about 0.1 μm are readily fabricated. Holes of 10 μm indiameter have been etched, and as fabrication equipment and techniquescontinue to improve, smaller diameter holes should also be realizable.This advantageously implies small capacitance associated with thesmaller diameter electrodes.

The columnar holes into or through the substrate are then madeconductive, to form collection electrodes, according to the presentinvention. For silicon substrate-based detectors, the column-like holesare preferably filled with silicon, for example as the result of asurface recombination with a silane gas. Using a silane process isadvantageous because the silane rebounds (or "bounces") off the siliconhole-wall surfaces many times before reacting, ensuring good silicondeposition throughout the length of the column-like holes.

Dopant gases preferably exhibiting similar behavior can be added to thesilane as the holes are filled. For example, diborane and phosphine,when added to the silane, allow the fabrication of P+ and N+ electrodesrespectively, e.g., P+ electrode 210 and N+ electrodes 220 in FIG. 2B.Silane, diborane, and phosphine each advantageously yield conformalcoatings without tending to close or clog the hole opening beforecovering the hole-column bottom region. In practice, silicon layersdeposited simultaneously on the wafer surfaces will have a thicknesssomewhat greater than the hole radius, and can be readily removed byetching.

In addition to experimenting to determine a smallest etch-formed holesize realizable with the equipment at hand, applicant investigatedpossible hole-filling electrode materials. Investigations wereundertaken to determine whether the hole could be filled withsingle-crystal silicon (epitaxial, or "epi"), or whether polycrystallinesilicon (polysilicon) should be used as fill material to formelectrodes.

Epitaxial material is generally more difficult to fabricate thanpolysilicon, but nonetheless ia may be possible to use epitaxial to fillclean electrode holes that were undamaged during etching, since theinterior hole surfaces are single crystal silicon. An epitaxial fillmaterial could advantageously be combined with a gradually increasingdopant level during deposition. Following annealing, a radial dopantgradient results, providing a radial built-in electric field. (Even withuniform doping, a dopant gradient will be present even after annealing,but the gradient will be smaller.) This field will transport ionizationcharges in the same direction as the applied field, providing rapidcharge collection from the entire volume of the detector, including theelectrodes.

Following a fill deposition, the silicon is heated so the dopant atomsmove to lattice sites and become electrically active. The dopant atomsalso diffuse out from the N+ electrodes into the P substrate bulk andform P-N junctions in high-quality silicon. By contrast, usingpolysilicon fill material, diffusion of dopant atoms follows grainboundaries and is far faster than in single crystal silicon. Thus, forpolysilicon fill electrode material, a nearly uniform doping density isestablished in the electrode. Unfortunately, such uniformity reduces themagnitude of the built-in field in the electrodes. Diffusion ofionization charge from the track to the start of an applied fieldseveral microns away, possibly with a small boost from Coulomb repulsionfrom the remainder of the track, appears necessary for collection.(Further details as to charge collection are provided later herein.)Recombination within the electrodes may not be a problem, asmeasurements made using 20 Ω-cm epi in a charge-coupled device ("CCD")vertex detector show diffusion lengths of about 200 μm.

Once electrodes, e.g., 210, 220, have been formed, fabrication steps canbe varied to produce monolithic pixel (or diode) detectors, bump-bondedpixel detectors, and strip detectors, with or without on-chip drivingelectronics associated with the bulk electrodes.

Although silicon is a most commonly used material for substrate 180,GaAs may be an even better candidate material. In general, largersubstrate thicknesses provide good X-ray and gamma-ray detectionefficiencies, but overly large thickness can introduce drift-lengthlimitations. But the present invention may advantageously provideelectrode spacings that are less than those drift length limitations,thus providing short maximum drift distances.

If the substrate material is GaAs, a material providing high electronmobility, the resultant detector 170 will provide very fast, probablysub-nanosecond, response times. Substrate-thick column-holes may beetched in GaAs and filled using metal organic chemical vapor deposition,for example with trimethyl gallium and arsine. A GaAs detector wouldcircumvent limitations on maximum drift distances associated with asilicon substrate, and contribute to an efficient X-ray detector.Because of the low depletion voltages associated with GaAs, one couldmaintain the electric field near magnitudes that give high driftvelocities, to achieve sub-nanosecond collection times. Such a GaAsdetector, if combined with pixel readout parallel processing couldhandle very high rates indeed.

FIG. 3A depicts equipotentials for one-quarter of the unit cell 200shown in FIG. 2B, with 10¹² substrate dopant atoms per cm³, and 5 Vapplied between the two metal electrodes. It is understood that what isshown in FIG. 31 (and other quarter-cell depictions herein) is a portionof cell 170 centered on P+ electrode 210, and showing portions of N+electrodes 220, 22, 224. In FIGS. 3A-3D, a solid heavy line depicts theN-P junctions, and a dashed line is used to depict the boundaries of thedepleted region.

The cylindrical electrode doping profile assumed for the configurationof FIG. 3A and indeed throughout this application is:

    10.sup.18 ·e.sup.-(r/r.sbsp.o.sup.).spsp.2

where r_(o) is elected to bring the dopant concentration to 10¹² /cc ata distance r=5 μm. This profile produces N+ electrodes having aresistance of about 3 Ω/μm, and P+ electrodes with a resistance of about7.5 Ω/μm.

FIG. 3B is similar to FIG. 3A except that the substrate dopantconcentration is raised to 10¹³ /cc. In FIG. 3C, dopant concentration isagain 10 ¹² /cc but the inter-electrode potential is raised to 10 V.FIG. 3D is similar to FIG. 3C except dopant concentration is raised to10¹³ /cc. The effects of surface charges are not included in FIGS.3A-3D. FIGS. 3A-3D do depict the absence of cylindrical symmetry in thefields and depletion depths into the electrodes. The lack of suchsymmetry is especially apparent for N+ electrodes 220, 224, which areadjacent to P+ electrode 210. Also apparent is the decrease in low-fieldvolume for the heavier substrate doping cases shown in FIGS. 3B and 3D.

FIG. 4 depicts the drift lines present in a quarter-cell for the case of10¹² /cc dopant concentration and 5 V potential. Again, a heavy linedenotes the N-P junctions, while a dashed line depicts the boundaries ofthe depletion region.

FIG. 5A depicts electric field magnitudes for a quarter-cell along linesfrom the P+ electrode 210 to an adjacent N+ electrode, electrodes 220 or224, for a doping concentration of 10¹² dopant/atoms per cc. Theelectric field magnitudes represent applied voltages of 50 V (uppermosttrace), 40 V, 30 V, 20 V, 10 V, 5 V, and 0 V (bottommost trace). FIG. 5Bis a similar representation except the electric field magnitudes arealong a line from P+ electrode 210 to diagonal N+ electrode 222. In FIG.5C, the electric field magnitudes are along a line from an N+ electrodeto an adjacent N+ electrode, e.g., from electrode 222 to 220, orelectrode 222 to 224.

In FIGS. 5A-5C, 5V (the next-to-bottom trace) is more than sufficientpotential to produce full depletion. Especially advantageously, peakelectric fields are more than an order of magnitude below typically100,000 V/cm avalanche field strengths.

FIG. 5D compares electric field magnitudes for a quarter-cell measuredalong a line from P+ electrode 210 to adjacent N+ electrode 220 or 224,e.g., for the case shown in FIG. 5A. In FIG. 5D, 10 V potential wasused, and the two curves represent substrate dopant concentrations of10¹² /cc and 10¹³ /cc. Note that the peak electric fields actuallydecrease as substrate doping increases by a factor of 10. The highersubstrate dopant level is somewhat analogous to what may occur withradiation damage. Peak fields, located where the depletion volume meetsthe electrodes, decrease due to the increase in voltage dropped acrossthe substrate, which is lightly doped compared to the electrodes. Thesmall but non-zero values of the electric fields at the ends of the plot(corresponding to the electrode centers) are due to approximations inthe finite element calculations used for the data plotted in FIG. 5D.

FIG. 6 depicts lines of equal drift time for potential distributions fora quarter-cell in which substrate dopant concentration is 10¹² /cc andapplied potential is 10 V. Zero time is measured from the P+ electrode(shown in upper left corner of FIG. 6) at a radius r=5 *m, and chargesare traced backwards. Due to the role played by diffusion, data lines inthe immediate vicinity of the zero-field points near the bottom center,and right center in FIG. 6 are not reliable. Further, relatively fewtracks are traced backwards to these zero-field regions.

However, FIG. 6 demonstrates that drift time from cell center is lessthan 1 ns, and that drift times from the other electrodes range from 1ns to 4 ns. The drift time from the far cell borders is infinity becausein those regions the collection field goes to zero. Thus, to obtainrealistic drift times for tracks in those regions, one should adddiffusion, and for ionization created near or inside electrodes, oneshould provide the built-in fields. The calculations depicted werecarried out using MEDICI.

FIG. 7A represents charge density contours (two contours per decade) forelectron-hole pairs, for a quarter-cell having 10 V potential, 10¹² /ccdopant concentration, for holes starting from the cell center at 0.1 ps.The contours in FIGS. 7A-7L were obtained by starting with an ionizationtrack of 24,000 electron-hole pairs parallel to the electrodes in aquarter-cell such as shown in FIG. 2B. In FIGS. 7A-7E, the ionizationtracks were parallel to the electrodes and went through the middle ofthe cell, which results should be typical of much of the area. In FIGS.7F-7L, the tracks were through the null point on the border between twocells, which should represent the slowest case.

FIG. 7B, and 7C are similar to FIG. 7A except that holes starting fromthe center are shown at 89 ps and 432 ps, respectively. In FIGS. 7D and7E, electrons starting from the cell center are shown at 89 ps and 432ps, respectively, again for 10 V potential and 10¹² /cc dopantconcentration. FIG. 7F is similar except that electrons starting fromthe null point are shown at 175 ps. In FIGS. 7G-7L, charge densitycontours are shown at 0.1 ps, 175 ps, 1.7 ns, 3 ns, 4 ns, and 5 ns,respectively; again for 10 V potential and 10¹² /cc dopantconcentration, for holes starting from the null point.

FIG. 8A shows current pulses seen on four collection electrodes (withthe P+ electrode pulse specifically labelled) from a track that isparallel to the electrodes and goes through the cell center. FIG. 8B issimilar but shows a track going through the null point between two N+electrodes. FIGS. 8A and 8B include the effects of induced pulses frommoving charges and diffusion, but do not include Landau fluctuations orCoulomb forces from the other charges along the track. In FIGS. 8A and8B, a 10 V potential was used, and dopant concentration was 10¹³ /cc.

The small difference in FIG. 8A (the midpoint start) between pulses onthe two N+ electrodes adjacent the P+ electrode is due to small butnon-zero grid effects. Note also that the effects of induced pulses frommoving charges can be seen. In FIG. 8A, the current pulse signal peaksat 0.5 ns and returns to the base line at 1.5 ns. This is in starkcontrast to the perhaps 25 ns to 40 ns return-to-baseline time requiredfor signals on strip detectors using prior art two-dimensionalelectrodes (such as shown in FIG. 1). The return-to-baseline time can beimportant for pile-up considerations, especially since Landaufluctuation effects can be present until all the charge is collected. InFIG. 8B, the current signal pulse on the P+ electrode for the null pointtrack peaks at 2.4 ns, and returns to the baseline at about 6 ns.

It is to be appreciated although the response times reflected in FIGS.8A and 8B are significantly shorter than typical times for detectorsusing two-dimensional planar electrodes, these shorter times areobtained with three-dimensional devices having far lower maximum fields.Thus better performance is attainable from devices operated at saferdevice electric field levels.

The various results thus far described pertain to charge motion in thesubstrate bulk of a detector having three-dimensional electrodes.However, close to the upper and lower surfaces of such a detector,consideration must be given to the effects of surface charges and indeedof the structure itself.

FIGS. 9A-9F depict calculated equipotentials obtained from a simpletwo-dimensional model in which the N+ and P+ electrodes were flat slabs.The slab electrodes were separated by a 15 *m region of silicon having10¹² acceptors/cc and covered with an oxide layer having 10¹¹ positiveinterface charges/cm². The electrodes were at 0.5 μm *m and 20 *m to 25*m, and were doped throughout at 10¹⁸ /cc. The charges were along thetop surface, between the electrodes.

FIG. 9A depicts data for 0 V applied potential. Note the effect ofnegative charge induced by the oxide charge, as manifested by theclosest equipotential line almost parallel to the surface. The contactof the induced charge with the N+ electrode to the right forcesequipotentials from the built-in field into a bundle next to the P+electrode. It follows that the capacitance between the two electrodeswill be relatively high. FIG. 9B shows similar equipotentials for 5 Vpotential, whereas FIG. 9C shows equipotentials for 10 V potential. Asthe applied potential is increased from 0 V to 10 V, an increasinglywider depletion zone at the surface is apparent. FIGS. 9D and 9E depictelectron density contours for 0 V and 10 V potential, respectively. FIG.9F depicts net carrier concentration at 0.1 *m below the surface for 0V, 5 V, 10 V and 20 V, with 10 V applied potential.

It is seen in FIGS. 9A-9F that a layer of induced electrons is presentthat nearly reaches the P+ electrode, and that application of a voltagebias to the electrodes causes a gap to appear. This result suggests thatuse of P+ guard rings around the top of P+ electrodes may not be neededin practice. However, such guard rings may be needed for radiationdamaged oxides with larger surface charges.

FIGS. 10A-10C depict top views, and FIGS. 10D-10H depict side views ofalternative detector structures, according to the present invention. InFIG. 10A, for example, P+ electrodes and N+ electrodes are shown, whichelectrodes may extend (into the page) through the entire thickness ofthe surrounding substrate (as do electrodes 210, and 220, for example inFIG. 2B). Alternatively, some or all of these electrodes may extend buta fraction into the substrate. In the embodiment of FIG. 10A, in a planview there are rows of N+ electrodes and rows of P+ electrodes, arrangedin a regular grid-like array. By contrast, in FIG. 10B, there are againrows of P+ and N+ electrodes, but instead of a grid-like array, adjacentrows are staggered or offset from each other. In the hexagon-shaped cellconfiguration of FIG. 10C, rows are arranged to include repeating groupsof two adjacent electrodes of one dopant polarity, one electrode of theopposing dopant polarity.

Several possible detector structures are shown in FIGS. 10D-10H, inwhich only two electrodes are depicted for ease of illustration. In eachof these figures, a portion of detector 170 is shown, in which detector170 is fabricated on a substrate 180 that includes a P- depleted region300 having an upper surface 190 and a lower surface 200. As seen fromFIGS. 2A and 2B, the distance between surfaces 190 and 200 defines thesubstrate thickness L. In each of FIGS. 10D-10H, the P+ doped electrodeis denoted 210, and the N+ electrode is denoted 240. Electrodes 210, 240are shown with a somewhat exaggerated lateral extension at upper surface190, the extensions pointing toward each other in these figures. It isbelieved that the extension surfaces can improve electrical contact tooverlying metal connecting traces (not shown), especially in view of anyuneven topography of the IC containing the various detectors.

In the configuration of FIG. 10D, an N-well 310 is implanted in a regionof the upper substrate surface 190 between the two electrodes, and an N+layer 320 is implanted in a region between the two electrodes onsubstrate lower surface 200. N+ layer 320 may advantageously be formedby driving phosphorus into single crystal silicon from a polysiliconlayer, thereby providing gettering from both the phosphorus andpolysilicon.

The configuration of FIG. 10E is somewhat different in that P+ rings330, 340 are formed in the region between the two electrodes at thesubstrate upper and lower surfaces. Note that oxide layer regions 350,360 separate and isolate the P+ rings from electrodes 210, 240. Theconfiguration of FIG. 10E (as well as that of 10D) would require backside lithography for fabrication. This is not difficult, however, inthat the structures are relatively crude and the hole-cylinders providealignment marks.

In the configuration of FIG. 10F, oxide layers 350 and 360 are formed inthe region between the two electrodes at the substrate upper and lowersurfaces, respectively. The embodiment of FIG. 10G provides an N-well310 at upper substrate surface 190 (as in FIG 10D), and provides anoxide layer 360 on the lower substrate surface 200. Note that electrodes210 and 240 are shown in FIG. 10G as having lengths less than thesubstrate thickness L. The embodiment of FIG. 10H replaces the oxidelayer 360 in FIG. 10G with an N+ layer 370. Note too that the L1, L2,lengths of electrodes 210, 240 are shown as being unequal. It isunderstood that electrode lengths according to the present inventionneed not be equal to each other, or to substrate thickness L.

It will be appreciated that the shortened electrode lengths shown inFIGS. 10G and 10H permit implanting a (conducting) N+ layer (e.g., layer370 in FIG. 10H) without requiring double-sided patterning. In general,cells with N+ layers are easier to deplete, and cells with P+ rings aremore difficult to deplete. On the other hand, however, the N+layer,forming an equipotential normally at the same voltage as the Nelectrodes will make regions with relatively slow drift velocities.(Upper surface wells in monolithic devices would also create suchregions.) In practice, these slower drift velocity regions ought not toaffect the main portion of a pulse from an ionizing particle, but couldadd somewhat to the tail portion of the pulse.

It is to be understood that in any or all of the configurationsdescribed herein, or fabricated according to the present invention, someor all of the electrodes may have a length L1 that is less than thesubstrate thickness L, and that the various electrode lengths need notbe equal.

FIG. 11 shows equipotentials for one cell of FIG. 10A, for 10¹² netdopant atoms/cc, and 10 V applied bias. FIGS. 12A depicts electric fieldmagnitudes for a cell such as shown in FIG. 10A along a line connectingan N+ electrode with a directly opposite (e.g., adjacent) P+ electrode.For the same cell, FIG. 12B depicts electric field potentials along amid-point horizontal parallel line through cell middle. (The tracevoltages are the same as shown in FIG. 5.)

FIG. 13 depicts a potential distribution for the hexagon cell of FIG.10C, for 10¹² dopant atoms/cc, and 10 V applied bias. If detector 170were fabricated using polysilicon electrodes 210 and/or 240, it might bebeneficial to provide such electrodes having lengths L1, L2<L. Thiswould be especially true if charge collection speed or efficiency forthe small number of tracks that are fully contained in the electrodeswas less than what is needed for a given application. In such anembodiment, substantially all tracks would generate signals in theregion of silicon below the electrodes. Alternatively, one couldfabricate two etched-through detectors that would then be glued togetherwith an offset.

Small wells that cover only a portion of the upper substrate surfacecould be provided to hold simple driving electronics for a fast readout,such as might be used in a mammography detector, among otherapplications (see FIG. 2A). A two-dimensional readout could be providedusing single-sided technology to fabricate electrodes that placedifferential signals on twin x and twin u or y lines that drivedifferential receivers. Advantageously, there would be negligible dangerof interference between crossing signals, given the double subtractionat the crossing and at the receiver. Signal height would be nearlyindependent of strip length, and incoherent noise would grow no fasterthan the square root of the length. For small-angle stereo, readoutwould be allowed only from the ends since u lines reaching one edgecould be crossed over the x lines and brought to the other edge, whencethey would continue at their stereo angle.

The role of charge collection from electrodes in the present inventionwill now be described. Charge from tracks contained within electrodeswill leave the electrodes by diffusion. In addition, if epi electrodesare used, charge will also leave because of the electric forces due tothe built-in fields. Coulomb forces within the ionization charge clouditself can either hinder or help charge collection, once part of thecharge has been collected. For example, in the case of floating N+electrodes, once some holes have diffused-out to the collection fieldand have been removed, the net negative charge will tend to attract theremaining holes, slowing their diffusion out of the electrode. If,however, the electrons have been collected by electronics coupled to theN+ electrode, there will be a net repulsion that will hasten holecollection.

Table 1, below, depicts pulse times for tracks at a radius r within anN+ electrode centered in a 50 *m×50 *m cell, a cell such as that shownin FIG. 2B for example. For polysilicon electrodes, the electric fieldwithin the electrode is assumed to come only from other holes (e⁻out--electrode coupled to electronics collecting the electrons) or fromboth holes and electrons (e⁻ in). Estimated errors in the times rangefrom 5% to 10%. For polysilicon electrodes, the time to collect 90% ofthe charge will be about three times that required to collect 50%.

                  TABLE 1    ______________________________________    track      1.0     2.0      3.0  4.0   5.0  μm    band    rmin       0.0     1.5      2.5  3.5   4.5  μm    rmax       1.5     2.5      3.5  4.5   5.0  μm    band area  0.283   0.503    0.754                                     1.005 0.597                                                %    cell area    solid angle/π               0.0025  0.0069   0.014                                     0.023 0.028                                                %    time-epi    peak       2.9     2.7      2.3  2.2   2.1  ns    .5 Σq               3.1     2.7      2.3  2.1   2.0  ns    time-poly, e out    peak       4.5     4.1      3.5  2.4   1.9  ns    .5 Σq               6.6     5.8      4.3  2.8   1.9  ns    time-poly, e in    peak       5.1     4.7      3.5  2.5   2.0  ns    .5 Σq               8.3     6.8      4.7  3.1   1.9  ns    ______________________________________

The fraction of tracks contained entirely within an electrode willdepend upon the tracks' angular distribution, as well as upon theelectrode cross-sectional area. Table 1 shows, for example, at row 4 thepercentage of a 50 μm×50 μm square cell occupied by radial bands for anN+ electrode between two p+ ones of FIG. 3C. Table 1 further shows theprobability for the track to remain inside the band outer radius, for abeam distributed uniformly over π steradians centered around the normalto the detector.

The probability for the entire track to stay within rmax is the productof the band area and solid angle factors. For a more tightly alignedbeam, the product increases to that of the area fraction alone, althoughfor very tightly aligned beams, the detector can be tilted, reducing thefully contained fraction to zero. The following two rows in Table 1 showthe times to the pulse peak and to the 50% charge collection time for 10V applied voltage and epi electrodes. The following two rows then showsimilar times for polysilicon electrodes (without the built-in fields),and with electrons collected by the attached electronics. Finally, thelast two rows depict data for floating polysilicon electrodes, with theelectrons left in the electrode. Tracks will also be contained withinthe P+ electrode and the two adjacent N+ electrodes, which will roughlytriple the area fractions given in Table 1. The times for theseelectrodes will be shorter than the times for the N+ electrodes,tabulated above.

Having described the present invention in detail, several generalconclusions and architectural design guidelines may be drawn. When it isnecessary to minimize electric fields, electrodes forming diodejunctions should have more total surface area than those forming ohmicjunctions. N+ electrodes with phosphorus doping serve as getters, andpreferably their electrode area is maximized, consistent with otherdesign requirements. P- substrate material is preferred to prevent typechange from bulk radiation damage, and indeed data exist indicating thatP- silicon is less subject to bulk damage than N- material. (Bulkdamage-induced type change may not be inevitably lethal in allimplementations, however.)

According to the above design guidelines, a P- substrate should be usedwith a P electrode that transmits signals from the entire pixel, whilePN diode junctions are formed between multiple N electrodes and thesubstrate. However, signals could be taken from the N electrodes,further subdividing the pixel and promoting faster signal collectionspeed. In practice, monolithic technology is likely to be needed for thesmallest readout pitches, e.g., spaced-apart distances. Withsufficiently fast electronic circuits, improved position and timeinformation may also be provided by comparing the various P electrodeand N electrode signal times and signal pulse heights.

In a bump-bonded pixel detector system, many N electrodes may be coupledtogether (e.g., with metal or diffusion conductive traces) to reduce thenumber of bumps required, and to provide a measure of redundancy. If ICchip area needed for on-chip electronics in the pixel causes pixel areato exceed the area of the underlying bulk cell, conductors may join Pelectrodes from one or more other cells to the pixel electronics.However, the penalties for so doing will include increased capacityΣC_(i), a reduced signal q/ΣC_(i), and a reduced signal-to-noise ratio.In such case, a single, simple front-end circuit per cell with an inputsignal q/C_(i) may be a better design choice. This design might bebetter because random noise of the sum increases, and signal-to-noiseratio decreases but at most only as the square root of the number ofcells. (For example, if one source-follower from each cell were used todrive a common pixel bus, a follower with a signal will tend to cut offthose without, and only the noise on that channel would be present.)

When on-wafer metal lines are used, preferably the outer faces of theelectrodes are in contact with, and surrounded by, an implanted ring oflike-polarity dopant to promote good contact. This practice is desiredas the silicon surface directly above the electrodes may not be fullyplanar.

Silicon surfaces may be inverted by charges in surface field oxidelayers. Thus, for a P-substrate, inversion could result in a continuousN conductor from the N electrodes to the immediate vicinity of the Pelectrodes. The resultant small gap could result in increased electrodecapacity and electric fields. On one hand, this gap would be enlarged bythe applied depletion voltage, but on the other hand, increased oxidecharge due to radiation damage might reduce the gap. To prevent this, P+guard rings may be formed around the P electrode or a blanket P implantmay be used. Implanted rings are recommended for use in test devices tomonitor surface leakage currents.

An exemplary sequence of process steps to produce bump-bonded diodesaccording to the present invention will now be given, in which routinewafer cleaning and process checking steps are not enumerated. Of course,even the enumerated steps may have many sub-steps, not listed here, andother fabrication steps and procedures may instead be used. For example,masking steps will involve spinning on photoresist, a low temperaturebake, exposure in a mask aligner, photoresist development, a hightemperature bake, the masked process per se (e.g., ion implantation oretching), and photoresist stripping. Even a simple sub-step such asspinning on photoresist will have its own sub-steps.

An exemplary fabrication process would include the following steps:

(1) Mask 1: alignment mark mask and etch;

(2) Mask 2: N electrode mask and wafer etch-through;;

(3) N+ silicon deposition and hole fill (e.g., using a silane/phosphinegas mixture)

(4) Etch back deposited silicon on both wafer surfaces

(5) Mask 3: P electrode mask and wafer etch-through

(6) P+ silicon deposition and hole fill (e.g., using a silane/diboranegas mixture)

(7) etch back deposited silicon on both wafer surfaces

(8) Thermal oxidation (e.g., 0.6 μm oxide thickness)

(9) Etch oxide,backside

(10) Backside blanket P+ implant (optional step, to prevent oxidecharges from inverting the adjacent silicon; alternatively, a maskingstep forming P+ rings to increase the N+/P+ separation could be used)

(11) Thermal oxidation (e.g., 0.6 μm oxide thickness, however ifoptional step 10 is omitted, steps 9 and 11 are also omitted.)

(12) Mask 4: front side N+ mask and implant to provide a planar ohmiccontact to the N+ electrodes as silicon fill at the electrode surfaceswill not necessarily be flat

(13) Mask 5: front side P+ mask and implant to provide planar ohmiccontact to P+ electrodes and surrounding guard rings if rings are usedto monitor current

(14) Anneal implants

(15) Low temperature oxide ("LTO") deposition

(16) Mask 6: contact mask and etch

(17) Aluminum deposition

(18) Mask 7: metal mask and etch

Modifications and variations may be made to the disclosed embodimentswithout departing from the subject and spirit of the invention asdefined by the following claims. For example, a GaAs detector fabricatedaccording to the present invention should realize collection times of 1ns or substantially less, maximum drift distances should no longer be asignificant limitation for such detectors.

What is claimed is:
 1. A radiation-damage resistant radiation detectorhaving non-planar electrodes, comprising:a substrate formed of amaterial doped with a first conductivity type dopant and having a firstsurface and a second surface spaced-apart by a substrate thickness of atleast about 50 μm; at least one first collection electrode formed ofsaid first conductivity type dopant and penetrating from a chosen one ofsaid first surface and said second surface into said substrate; and atleast one second collection electrode formed of a second conductivitytype dopant and penetrating from a chosen one of said first surface andsaid second surface into said substrate; at least one of said firstcollection electrode and said second collection electrode penetratinginto said substrate a distance exceeding about 5% of said substratethickness; said first collection electrode and said second collectionelectrode being spaced-apart from each other such thatradiation-released holes and electrons traverse a collection distanceless than said substrate thickness to be collected by at least a portionof one of said first collection electrode and said second collectionelectrode.
 2. The detector of claim 1, wherein at least one said firstcollection electrode and said second collection electrode has a dopantprofile graded radially in a plane parallel to said first surface ofsaid substrate.
 3. The detector of claim 1, wherein at least one of saidfirst collection electrode and said second collection electrode isformed from at least one material selected from a group consisting of(i) polysilicon doped with a chosen conductivity type dopant, (ii)epitaxial silicon doped with a chosen conductivity type dopant, (iii)trimethyl gallium and arsine, and (iv) dopants appropriate for asubstrate other than silicon or gallium arsenide.
 4. The detector ofclaim 1, wherein at least one of said first collection electrode andsaid second collection electrode penetrates into said substrate adistance equal to 100% of said substrate thickness.
 5. The detector ofclaim 1, wherein said first collection electrode and said secondcollection electrode have a spatial relationship selected from a groupconsisting of (i) said first collection electrode and said secondcollection electrode each penetrate into said substrate from oppositesurfaces of said substrate, (ii) said first collection electrode andsaid second collection electrode each penetrate into said substrate froma common surface of said substrate, (iii) said first collectionelectrode penetrates more deeply into said substrate than does saidsecond collection electrode, (iv) said first collection electrodepenetrates from said first surface through said substrate into saidsecond surface, and (v) said first collection electrode and said secondcollection electrode each penetrate from said first surface through saidsubstrate into said second surface.
 6. The detector of claim 1, whereinsaid first collection electrode and said second collection electrode areseparated by a spaced-apart distance having at least one attributeselected from a group consisting of (i) said spaced-apart distanceranges from about 0.1 μm to about said substrate thickness measured fromcollection electrode center-to-center, and said spaced-apart distance issufficiently small to permit radiation-released charge in said substrateto be detected within a time ranging from about 0.01 ns to about 10 ns.7. The detector of claim 1, wherein when an externally applied potentialis coupled between said first collection electrode and said secondcollection electrode, said potential affects said detector in at leastone manner selected from a group consisting of (i) said potentialpromotes depletion of said substrate in addition to substrateself-depletion, (ii) said potential increases magnitude of an electricfield present between said first collection electrode and said secondcollection electrode, and (iii) said potential promotes an electricfield that guides radiation-released charge in said substrate to one ofsaid first collection electrode and said second collection electrode forcollection;wherein said externally applied potential has a magnituderanging from 0 VDC to about 100 VDC.
 8. The detector of claim 1, whereinspaced-apart spacing between said first collection electrode and saidsecond collection electrode is such that a radiation-detected signalbetween said first collection electrode and said second collectionelectrode has a non-zero time duration of less than about 5 ns.
 9. Thedetector of claim 1, wherein said first collection electrode defines acolumnar shape having a transverse dimension ranging from about 0.5 μmto about 25 μm.
 10. The detector of claim 1, wherein if said firstconductivity type dopant is P-type then said second conductivity typedopant is N-type, and if said first conductivity type dopant is N-typethen said second conductivity type dopant is P-type.
 11. The detector ofclaim 1, wherein said substrate includes at least one material selectedfrom a group consisting of (i) silicon, (ii) germanium, (iii) galliumarsenide, and (iv) cadmium zinc telluride.
 12. The detector of claim 1,wherein said substrate thickness is about 10 mm.
 13. The detector ofclaim 1, wherein at about 20° C. ambient, following at least onecondition selected from a group consisting of (i) a net imbalance ofradiation-induced acceptor, and (ii) donor sites exceeding about 10¹²/cm³ to about 10¹³ /cm³, said radiation detector continues to detectwithout requiring application of an external substrate depletion voltageso large in magnitude as to cause said detector to experience acondition selected from a group consisting of (a) excessive leakagecurrent, and (b) voltage breakdown of said substrate.
 14. The detectorof claim 1, further including means for reading-out radiation detectedby said detector, said means for reading-out having a configurationselected from a group consisting of (i) said means for reading-out isfabricated as a separate integrated circuit that is bump-bonded to asurface of said substrate, (ii) said means for reading-out is fabricatedas a strip on a surface of said substrate, (iii) said means forreading-out and said detector are fabricated as a monolithic detectionunit, and (iv) said substrate includes an isolation well in which saidmeans for reading-out is fabricated.
 15. The detector of claim 1,further including a first plurality of said first collection electrodesand a second plurality of said second collection electrodes, said firstand second plurality defining a regular pattern on a plane parallel tosaid first surface of said substrate.
 16. A radiation-damage resistantradiation detector having non-planar electrodes, comprising:a substrateformed of a material doped with a first conductivity type dopant andhaving a first surface and a second surface spaced-apart by a substratethickness; a first plurality of first collection electrodes formed ofsaid first conductivity type dopant and penetrating from a chosen one ofsaid first surface and said second surface into said substrate adistance ranging from about 5% to 100% of said substrate thickness; anda second plurality of second collection electrodes formed of a secondconductivity type dopant and penetrating from a chosen one of said firstsurface and said second surface into said substrate a distance rangingfrom about 5% to 100% of said substrate thickness; said first collectionelectrodes and said second collection electrodes being spaced-apart fromeach other such that radiation-released holes and electrons traverse acollection distance less than said substrate thickness to be collectedby at least a portion of one of said first collection electrode and saidsecond collection electrode; wherein at least one of said firstcollection electrode and said second collection electrode has a dopantprofile graded radially in a plane parallel to said first surface ofsaid substrate.
 17. The detector of claim 16, wherein said firstcollection electrodes and said second collection electrodes have aspatial relationship selected from a group consisting of (i) said firstcollection electrodes and said second collection electrodes eachpenetrate into said substrate from opposite surfaces of said substrate,(ii) said first collection electrodes and said second collectionelectrodes each penetrate into said substrate from a common surface ofsaid substrate, (iii) some said first collection electrodes penetratemore deeply into said substrate than do some said second collectionelectrodes, (iv) some said second collection electrodes penetrate moredeeply into said substrate than do some said first collectionelectrodes, and (v) all said first collection electrodes have a firstelectrode length and all said second collection electrodes have a secondelectrode length, wherein a ratio of said first electrode length to saidsecond electrode length has a characteristic selected from a groupconsisting of (a) said ratio=1, (b) said ratio>1, and (c) said ratio <1.18. The detector of claim 16, further including:an isolation wellfabricated in said substrate; and read-out circuitry, fabricated in saidisolation well, coupled to an end portion of each of said firstcollection electrodes and said second collection electrodes; whereinradiation-released charge collected by any of said first collectionelectrodes and said second collection electrodes are read-out by saidread-out circuitry.
 19. The detector of claim 16, wherein:said firstcollection electrodes have a columnar shape with a transverse dimensionranging from about 0.5 μm to about 25 μm; and closest ones of said firstcollection electrodes and said second collection electrodes arespaced-apart from each other a distance ranging from about 0.1 μm toabout said substrate thickness measured from collection electrodecenter-to-center.